Foundations of Linux ARM64: Debug, Disassemble, and Reverse/

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Assigning Numbers to Memory Using Pointers

Assigning Numbers to Memory Using Pointers

Learn how to assign numbers to memory locations using pointers.

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Pointers to assign numbers

The following sequence of pseudocode instructions means that we interpret the contents of the x0 register as the address of a memory cell and then assign a value to that memory cell:

C++
x0 <- address a
[x0] <- 1

In the C/C++ language

In C and C++, it is called dereferencing a pointer:

C++
int a;
int *pa = &a;  // declaration and definition of a pointer 
*pa = 1;       // get a memory cell (dereference a pointer)
               // and assign a value to it

In the assembly language

In the assembly language, we write the following:

C++
adr x0, a     // load the address “a” into x0
mov w3, #1    // set the temporary register to 1
str w3, [x0]  // use x0 as a pointer and store 1 at the memory address in x0

In GDB disassembly output

In the GDB disassembly output, we see something like this:

C++
0x00000000004000b0 <+0>:     adr     x0, 0x4100f0
0x00000000004000b4 <+4>:     mov     w3, #0x1
0x00000000004000b8 <+8>:     str     w3, [x0]

Explanation

To illustrate some instructions and not to be dependent on how the compiler translates C/C++ code, we wrote the program in the assembly language.

Source code

The source code of the pointers is given below:

Assembly (GAS x86)
.data
a: .word 0
b: .word 0
.text
.global _start
main:
_start:
	adr  x0, a
        mov  w3, #1
        str  w3, [x0]
        adr  x1, b   
	str  w3, [x1]
	ldr  w2, [x0]		
	ldr  w3, [x1]
        add  w4, w3, w2
        str  w4, [x1]
        add  w2, w2, #1
        str  w2, [x0]
	mul  w3, w4, w2
	str  w3, [x1]
	
	mov  x0, #0
        mov  w8, #93
        svc  #0

Here is a breakdown of the code presented above:

  • Lines 1–11: These are the header commands of the assembly code.
  • Line 12: We load the address of a and store it in x0 register.
  • Line 13: We assign #1 to the register w3.
  • Line 14: Then, we store the value from register w3 and assign it to the address pointed by x0.
  • Line 16: We load the address of b and store it in x1 register.
  • Line 17: We store the value from register w3 and assign it to the address in x1.
  • Line 19: We load the value in [x0] and store it in the w2 register.
  • Line 20: We load the value in [x1] and store it in the w3 register.
  • Line 21
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